feat: #3606679 Add hook_quicktabs_tab_content_alter()

New alter hook that fires after tab content is rendered, allowing modules to modify the render array before output. Fires in three places: the renderer for the default tab on AJAX instances, the renderer for all tabs on non-AJAX instances, and the AJAX controller for lazily loaded tabs. Parameters: &$render, $tab, $instance.

Closes #3606679